Filters are known that employ an outer filter body holding disposable filter cartridges, the outer body usually has a cylindrical housing and a cap, a coaxial return pipe usually has a threaded end and the cap screws onto the threaded end to clamp the cartridges in operative position in the housing. Liquid flows though an inlet, through filter media in the cartridges and out through the return pipe. Spent cartridges are simply exchanged with new ones. Typical cartridges are known that are made from a paper roll held in a plastics or metal cylinder. An example is described in WO/0037232. There are various alternative constructions and flow modalities used.
European patent application No. P 0 556 506A1 describes a modular filter system consisting of cartridges made from a tightly wrapped paper filter media retained in a plastic casing. The plastic casing has integrally formed grate-like configuration on its top and bottom surfaces to allow fluid to enter an exit the filter. The casing is also dimensioned to closely surround the paper media. The filter cartridges are stackable within the filter housing which can retain a plurality of cartridges thereby increasing the capacity of the filter. Each casing is made from identical top and bottom halves where the cylindrical walls clip together around the paper roll. When two cartridges are used the grid like top of one cartridge a butts the grid-like bottom of the other so that liquid flows from one cartridge to the next in series. Thus liquid flows one way, from one end only and is recovered out the other end. The central pipe in this case is an inlet pipe rather than a return pipe.
Other arrangements are known where liquid flows simultaneously into the filter in opposite directions and is recovered centrally, see for example Australian Patent 729523 and U.S. Pat. No. 4,792,397.
Australian Patent 729523 and WO/0037232 describe an injection moulded filter cartridge filled with a paper roll filter media. Unlike the cartridge of European patent application No. P 0 556 506A1, the cartridge is open ended and does not employ grate-like ends. This allows a press to be used to insert paper rolls under pressure. In one cartridge opposite ends of the cartridge are completely open so the cartridge is effectively a tube with two back-to-back paper rolls separated by gauze providing a return flow path. The paper rolls have a central bore made of cardboard for a return pipe. Thus when the cartridge is clamped inside a filter container liquid flows from an inlet in the container in both ends of the cartridge, through the paper rolls and is recovered by flowing out the ends of the paper rolls through a common return path formed by the gauze to the central return pipe. The gauze separates the confronting ends of the rolls and provides a flow path for the filtered liquid. Gauze is also used in the arrangement of Australian patent 659793 to separate two paper rolls in a metal cannister.
In U.S. Pat. No. 4,792,397 back-to-back paper rolls are used in a similar arrangement Australian Patent 729523 and WO/0037232 except instead of gauze being used to separate the rolls U.S. Pat. No. 4,792,397 uses a specially made fluid collector between the rolls. The fluid collector has closely spaced concentric ridges projecting from opposite sides of a substantially flat portion, pressure equalisation holes pass through the flat portion. The ridges push into the paper rolls on opposite sides of the collectors. Gaps in each ridge allows radial return flow to the central return pipe.
In Australian patent 650176 a metal canister is provided with end wall depressions to improve radial return flow. In Australian patent application 59243/00 a fluid collector is provided in an end wall of the housing, the fluid collector has lands in the form of sectors of a circle separated by radial flow passages.
The prevailing design criteria adopted in all these instances is to support the paper across its ends in an effort to maintain the return flow and minimise blockages that might impede flow as the paper collapses. Since the filters are pressurised as the filter media ages and becomes loaded it may distort in the region of the return flow leading to blockages. The gauze may become depressed by the paper being forced across the gap. Thus in the above examples it is usual to provide a substantial amount of support at the end of the paper roll with fluid collectors with relatively large flow paths being preferred to wire gauze.
The present invention adopts a completely different approach to the problem of supporting the roll while maintaining greater efficiency of return flow across the filter.